1. Field of the Invention
This invention relates to an ignition timing control system for internal combustion engines, and more particularly to an ignition timing control system of this kind which is adapted to control the ignition timing of an internal combustion engine provided with a plurality of changeover devices having influence upon the output torque of the engine.
2. Prior Art
Conventionally, an internal combustion engine is known, which is associated with changeover devices, such as a transmission the reduction ratio of which can be changed to different ratios, and a valve timing selector device for changing the valve timing of intake valves of the engine, i.e. valve opening duration and/or valve lift thereof.
In an internal combustion engine provided with such changeover devices, there is a problem that a torque shock can arise when such a changeover device effects changeover of the operative state thereof. To suppress the torque shock, it has already been proposed to employ a technique of retarding the ignition timing, e.g. by Japanese Provisional Patent Publication (Kokai) No. 63-36024.
However, when more than one changeover device have almost simultaneously effected changeover of the operative states thereof, a final correction value which is the sum of a plurality of correction values for retarding the ignition timing calculated in response to changeover of the operative states of the respective changeover devices, is virtually applied to retard the ignition timing. As a result, the ignition timing can be retarded so excessively that a misfire may take place to degrade the driveability of the engine.
Further, the ignition timing can be also retarded to reduce a torque shock when the engine is accelerated from a low speed. If a correction value for retarding the ignition timing is added to the above-mentioned sum of the correction values, the ignition timing is further retarded, which makes the engine further liable to the above-mentioned inconveniences.